1. Field of the Invention
The present invention relates to a method for fabricating a power device, and more particularly to a method for fabricating a bipolar power transistor.
2. Description of Prior Art
A prior-art bipolar power transistor shown in FIG. 1 comprises a heavily n-doped semiconductor substrate 1 for forming the collector of the bipolar power transistor; a n-type epitaxy layer 12 formed on the heavily n-doped semiconductor substrate 1; a p-type epitaxy layer 14 formed on the n-type epitaxy layer 12 for forming the base of the bipolar power transistor; a lightly n-doped region 16 formed on the p-type epitaxy layer 14; and a heavily n-doped region 18 formed on the lightly n-doped region 16 for forming the emitter of the bipolar power transistor. For increasing the current gain of the bipolar power transistor mentioned above, the emitter (i.e. the heavily n-doped region 18) is formed on the lightly n-doped region 16. However, due to a larger resistance provided in the lightly n-doped region 16, the secondary breakdown effect and the thermal effect will appear to reduce the switching speed of the bipolar power transistor when it is operated at a high current.